The invention relates to a bipolar plate and a fuel cell unit, and to a fuel cell that includes such a bipolar plate.
For mobile applications of fuel cells it is necessary to reduce the volume and size of the fuel cells (which are assembled as part of fuel cell stacks), to accommodate the narrow installation space conditions in vehicles, and to optimize the power density. The volume of the fuel cell stack is defined in essence by the height and/or the thickness of the bipolar plates, which are assembled as part of the fuel cell stack. For mobile applications they typically exhibit a thickness ranging from 0.9 to 1.2 mm. For non-mobile applications an even greater thickness is possible, since in this case the power density of the fuel cell stack is less relevant.
A typical thickness of a membrane electrode assembly (MEA) of the fuel cell is approximately 0.5 mm. Thus, approximately 65% of the cell block height of the fuel cell stack is defined by the height of the bipolar plates. Accordingly, attempts have been made to reduce even further the height of the bipolar plates, particularly in the area of the so-called flow field channels which, for example, convey the reaction fluids planarly to the membrane in the MEA. The cooling fluid carries away the reaction heat from the membrane area.
One difficulty encountered in reducing the height of the bipolar plates is that it is also necessary to reduce the installation height in the inflow areas of the fluids from the edge-sided ports to the actual fluid channels in order to be able to reduce the height of all of the bipolar plates. On the one hand, the inflow area should occupy as little space as possible; but, on the other hand, they should be large in order to guarantee a uniform distribution of the fluids. At the same time the inflow area must exhibit a very high rigidity and guarantee support for the MEA. This presents a problem for embossed bipolar plates, which are made, for example, of thin metal plates, because the fluids cross each other in the inflow area. This means that the height of the inflow area has to be reduced even further.
German patent document DE 100 150 360 A1 discloses a bipolar plate which achieves an intersection of fluids in thin bipolar plates so that a cooling fluid is conveyed obliquely over a right angled structure of the gas conveying channels. In this case the depth of the channel on the anode and cathode side can be reduced in the areas, in which the cooling fluid intersects.
Published U.S. Patent Application No. 2002/0081477 A1 discloses additional ways in which fluids can be distributed transversely, for example in a flow field, with parallel fluid channels. In this case the height of the opposite fluid channel is trimmed so that a transverse connection can be realized. However, there is the particular problem that it is necessary to have not only a gas distribution but also an additional cooling fluid distribution in the fluid channels.
One object of the invention is to provide a bipolar plate which has a negligible installation height and at the same time is especially rigid.
Another object of the invention is to provide such a bipolar plate which is especially suitable for the distribution of three fluids.
Similarly another object of the invention is to provide a fuel cell stack having a negligible installation height.
These and other objects and advantages are achieved by the bipolar plate according to the invention, which comprises two sub-plates, at least one of which has a uniform arrangement of raised, positive support points in the manifold zone and/or accumulation zone of said sub-plates. Apart from the support points situated near edges, a negative support point of the same type is designed adjacent to each raised positive support point inside the manifold zone and/or the accumulation zone. Such negative support point forms a free space for the fluid between the positive support points. It has been demonstrated that the regular configuration of support points and free spaces, for a uniform distribution of a fluid that is flowing through into the fluid channels of the bipolar plate, is especially advantageous. Furthermore, the support effect of the sub-plates in relation to each other is improved while at the same time optimizing the free spaces for the flowing fluid. Especially in the case of thin, embossed metallic bipolar plates and/or sub-plates the results are outstanding rigidity and support.
A pressure loss can be minimized if the support points are designed, according to an advantageous further development, as round or semi-spherical nubs. Due to the rounded shape it is possible to gently divert the flow. The nub shape represents an optimal contour from a flow engineering viewpoint. Furthermore, the shape is optimal for an embossing technique that is advantageous for the production of the thinnest stable bipolar plates. Due to the improved stability and/or rigidity of the embossed bipolar plate there is no need for an additional component to support the fuel cell membrane and/or the membrane electrode unit.
In a simplified embodiment of the invention, the support points exhibit a longitudinal, approximately elliptical cross section. Due to the resulting longitudinal characteristic of the support points and free spaces, the flow is advantageously in the preferred direction. Preferably adjacent support points are arranged at a varying angle to the fluid channels, in such a manner that the flowing fluid experiences a deflection in a preferred direction. This design is especially suitable for fluid ports at the corners of the bipolar plates, since an enhanced distribution of especially the most remote fluid channels can be achieved. It is desirable to assign the ellipsoid support points to the fluid ports near the corners.
If the directly adjacent positive and negative support points exhibit at least one continuous flank and form a honeycomb structure, the result is an especially rigid bipolar plate that is easy to emboss. In the case of a honeycomb structure the positive and negative nubs pass directly over into each other. In the extreme case, if it were not necessary for the edges to have a radius, the nubs in this embodiment would assume an octagonal cross section. Four sides would merge into a slope, the other end of which would pass over into the respective adjacent nub with the opposite (negative) orientation. The other four sides would be adjacent to a slope, the other side of which would pass over into a plane on the low level and would rise again to the next nub.
There is an additional improvement in the equipartition if the support points and the free spaces exhibit a higher flow resistance in the fluid port vicinity of an assigned fluid port than at a distance from the fluid port. Thus, the support points may be designed, as seen in the flow direction, with a larger diameter in the fluid port vicinity than at a distance from the fluid ports. As an alternative or in addition, the support points may be arranged, as seen in the flow direction, tighter in the fluid port vicinity than at a distance from the fluid ports.
An improvement in the equipartition of the fluids may be achieved if the flow of the fluid in the manifold zone and/or in the accumulation zone forms in essence a cross flow in relation to the fluid channels. The manifold zone and/or the accumulation zone advantageously taper off in the direction of flow.
If the manifold zone and the accumulation zone are asymmetrical to each other, the result is a uniform distribution of the respective fluid among the fluid channels. Fewer support points in the manifold zone and/or accumulation zone are necessary—for example, nubs between the sub-plates and between the fuel cell membrane and/or the MEA and the bipolar plate. The mechanical stability is increased. Furthermore, a better water discharge from the fuel cell stack is possible—for example, at a cold start of the assigned fuel cell system. The manifold zone and the accumulation zone should occupy as little area as possible, since they are usually not a part of the electrochemically active area of the fuel cell stack, and thus have a negative impact on the power density.
It is especially advantageous if the manifold zone and the accumulation zone are different in size, and if the manifold zone occupies less area on the sub-plate than the accumulation zone.
One advantageous further embodiment of the invention provides that at least one of the sub-plates has a passage opening, which makes it possible for the fluid to pass between the interior, enclosed by the sub-plates, and the respective flat side. A partially closed fluid guidance enables a compact design and maximum utilization of the installation height. The introduction of the fluids from the fluid port to the manifold zone and/or the discharge of fluids from the fluid channels of the flow field areas to the accumulation zone can take place without influencing the cross section of the channel and without having a negative effect on the fluid separation.
The passage opening is provided advantageously between the fluid port area and the manifold zone and/or between the accumulation zone and the exit-sided fluid port area.
In another advantageous embodiment, the at least one fluid port is surrounded by a circumferential sealing groove. In this way, with the configuration of circumferential sealing grooves on all existing fluid ports, it is possible to offset the grooves in the region of the desired fluid feed between the two sub-plates. It is desirable to select the offset as a function of the depth of the channel in order to guarantee a channel cross section that meets the minimum requirement.
If the sealing groove of the fluid port on the one sub-plate extends, at least in certain places, inside the sealing groove of the respective fluid port on the other sub-plate, then it is possible to guarantee sufficient tightness when the fuel cell stack is assembled, since the sealing grooves on the one sub-plate can be supported by the adjacent sub-plate and vice versa.
The sealing groove of the one sub-plate is supported advantageously by a support structure of the other sub-plate. The mechanical support of the sealing grooves occurs in an optimal manner by means of a structured adjacent sub-plate. This can be done especially advantageously with embossed structures. A good compromise can be found between the mechanical stability and a channel cross section that meets the minimum requirement.
In a further embodiment, a reliable separation of the fluids is achieved by locating a weld joint for joining the two sub-plates outside an area that is enclosed by the fluid port and the passage opening. Furthermore, there is no adverse effect on a circumferential sealing groove. It is practical to arrange the weld joint adjacent to the passage opening in the direction of the interior of the sub-plates.
A fuel cell stack according to the invention comprises a layered arrangement of fuel cells, which are separated by bipolar plates, and has at least one bipolar plate having one or more of the above described features.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.